Traditionally natural gas is compressed and sent down pipelines under high pressure to transport the gas to market. When the gas has arrived at its use point, the pressure of the natural gas is reduced in one or more control valves to its final pressure for consumption. The available energy from the reduction in pressure of the natural gas is wasted in the control valves as well as any chilling effect (also known as the Thompson effect) caused by the flow of natural gas through these devices.
In the past, advantage has been taken of this wasted energy by facilities utilizing the energy and refrigeration effect of expanding the natural gas. One such facility was designed and constructed by Airco Industrial Gases' Cryoplants Division in the early 1970's in Reading, Pa. for UGI Corporation. It employed a natural gas pressure reduction station (“Letdown Station”) to make liquefied natural gas (“LNG”) or liquid nitrogen (“LIN”). A majority of the natural gas entering the plant under high pressure from the transportation pipeline was cooled and sent to an expansion turbine where energy and refrigeration were generated. The remainder of the stream was subsequently cooled with the refrigeration and a portion liquefied. The liquefied portion was then passed to a storage tank as LNG product. The natural gas that was not liquefied was warmed, collected and sent to the low pressure main at a lower pressure than the high-pressure main.
U.S. Pat. No. 6,196,021 describes a system that uses natural gas expansion to provide refrigeration to liquefy a natural gas stream which is then revaporized by heat exchange with a nitrogen stream to cool the nitrogen stream. This refrigeration supplements refrigeration provided by nitrogen pressure letdown and a nitrogen cycle to provide liquid nitrogen. Similarly, U.S. Pat. No. 6,131,407 describes a system that produces LIN to be sent directly to an air separation unit (“ASU”) to assist refrigeration of the ASU. U.S. Patent Application Publication No. 2014/352353 describes a similar system to the system of disclosed by U.S. Pat. No. 6,131,407, but adds that the LIN produced can be sent to a tank instead of directly to the ASU. In each of these system, LNG is revaporized to provide for nitrogen cooling. However, it is not desirable to liquefy and then revaporize the natural gas, as this is thermally inefficient. U.S. Pat. No. 6,694,774 describes a system that uses natural gas letdown to provide refrigeration to produce a liquefied natural gas stream, where the refrigeration is supplemented by a closed loop mixed refrigerant cycle.